The stators and rotors of variable reluctance machines have magnetic saliencies commonly known as salient poles. Such a configuration is commonly called "doubly salient" as illustrated in FIG. 1. Each stator pole 2 is surrounded by a winding of one or more turns of electrically conductive material and appropriate insulation. A phase winding 3 is a pair of series connected windings respectively wound on diametrically opposed poles 2. Only one phase winding 3 is illustrated with it being understood that the remaining pairs of poles each have a phase winding wound on them. The phase windings 3 are grouped together so that a balanced torque is produced in the machine when the windings are excited from an external source of electrical energy and also so that voltage and current requirements of the external source are satisfied. There are no windings of any type or magnets associated with the machine rotor 4. The number of poles on the stator 5 differ from the number of poles on the rotor 4. When the rotor 4 is rotated with respect to the stationary stator poles 2, a variation in reluctance is observed in stator poles. This variation in reluctance is observed as a variation in the inductance of the phase windings 3 which can be readily measured by appropriate instrumentation. Starting from the condition of a stator pole 2 being exactly half way between two rotor poles 6, known as the "unaligned position", the inductance of the phase winding 3 has its minimum value. The unaligned condition in most variable (switched) reluctance machines generally exists throughout an arc of several degrees of rotor rotation. The inductance of the phase winding 3 is fairly constant at its minimum value throughout this arc. Excitation of the phase windings 3 during this rotation period of constant, minimum inductance results in negligible developed torque. As the rotor 4 turns beyond the arc of minimum inductance, the inductance measured in the phase winding 3 increases to a maximum value, which is when a pair of rotor and stator poles 6 and 2 are exactly aligned, known as the "aligned position" as illustrated in FIG. 1. When the stator winding 3 is excited with an electrical current as the inductance is increasing from minimum to maximum motor torque is developed on the machine shaft 7. When the phase winding 3 is excited as the inductance is decreasing from maximum to minimum, torque of the opposite direction is developed on the shaft 7. This torque is often termed "generator torque" or "regenerative torque", the latter term being associated with a motor in a braking mode. The dotted line 8 illustrates the electromagnetic field flux linkage through the rotor 4, rotor poles 6, stator poles 2 and stator core 9 in the minimum reluctance position. The electromagnetic flux does not have an appreciable component in axial direction parallel to the machine shaft 7.
In modern variable reluctance machines, switching of the phase windings 3 is accomplished by solid state switching devices, generally known as power semiconductors. Specific switching devices include thyristors, transistors, MOSFETs, IGBT's, and many other devices including combinations of the abovementioned devices. In general, the power semiconductors are operated in an "on/off" mode rather than a linear mode associated with linear amplifiers. The switching times of the power semiconductors are determined by a controller. The controller operates in response to various sensors which sense such machine parameters as the position of the rotor poles with respect to the stator poles, current levels in the windings, voltage levels, or other signals required for the desired operation and protection of the machine.
Variable reluctance motors exist which utilize permanent magnets to provide holding torque. See U.S. Pat. Nos. 3,984,711, 4,048,531, 4,712,028 and French Patent 2,315,793. The systems disclosed in U.S. Pat. Nos. 3,984,711 and 4,712,028 have permanent magnets disposed within the slots of the stator. The magnets disclosed in the '711 patent and '028 patent within the slots of the stator reduce the efficiency of the motor from the standpoint of providing maximum torque and power by reducing the space which could be utilized for electrical windings to produce electromagnetic flux which produces motor torque. The motors disclosed in the '711 and '028 patents, have the magnets located in a high temperature zone as a consequence of the heat generated by the windings within the stator slots which can reduce the flux of the permanent magnets over their useful life. Finally, the stator structure of the '711 and '028 patents is complicated in that the windings and permanent magnets must both be disposed in the slots including structures to mount the permanent magnets permanently within the stator slots. Finally, the location of the magnets on the face of the salient poles of the stator of the '028 patent can cause interference with the magnetic field linkages with the rotor during normal motor operation. The motor disclosed in U.S. Pat. No. 4,048,531 utilizes a pair of rotors and stators with a second stator having a permanent magnet which generates a magnetic field used for generating holding torque. The structure of the '531 patent is complicated in requiring multiple stators and rotors. The variable reluctance stepping motor disclosed in French Patent 2,315,793 is a mechanically complicated structure with the rotor offset axially from the control windings which results in an elongated motor structure. The permanent magnet utilized in the '793 patent is axially aligned with the stator windings at a radius greater than the stator windings.
U.S. Pat. No. 4,827,164 discloses a stepping motor with salient stator poles. The salient stator poles have permanent magnets mounted on arcuate surfaces facing the rotor. The permanent magnets develop holding torque.